1. Technical Field
The present application relates generally to electronic ballasts and, in particular, a single stage feedback power factor inverter which prevents a direct current DC bias from being applied to the resonant circuit current in a triac dimmable electronic ballast.
2. Description of Related Art
Typically, a conventional electronic ballast is implemented using feedback connections from a high frequency resonant circuit to a node between an alternating current (AC) rectifier and an isolating diode through which current to a direct current (DC) energy storage capacitor flows. The stored DC energy is converted into a square wave voltage waveform by an inverter which is driven, for example, by a drive control circuit.
For example, U.S. Pat. No. 5,404,082 to Hernandez et al. entitled "High Frequency Inverter with Power-Line-Controlled Frequency Modulation" discloses a low-cost electronic ballast for use with fluorescent lamps which utilizes a single stage feedback inverter topology. As is understood by those skilled in the art, the inverter is typically implemented as a half-bridge inverter using a pair of switches, e.g., MosFets. During steady state operation, for example, it is desired that these switches operate in what is known in the art as "zero voltage switching," which is a term that refers to operating the switches in an inductive mode. In an inductive mode, the current flowing in the resonant inductor (choke) lags the voltage across the inverter (i.e, resulting in an inductive load). On the other hand, when the current flowing through resonant inductor leads in phase the voltage across the inverter, the inverter is said to be operating in a capacitive mode. When operating in a capacitive mode, the switching losses are significant and severe damage can occur to the switches as is understood by those skilled in the art.
Referring now to FIG. 1 a block diagram illustrates an embodiment of a conventional single stage feedback inverter topology of an electronic ballast utilizing triac dimming. The electronic ballast includes an alternating current source 10 (e.g., standard AC line voltage of 120 volt and a frequency of 60 hz) which provides input power for operating the ballast. A triac switch 12 provides phase angle dimming of the electronic ballast by cutting off a portion of the phase of the AC input line voltage (depending on its setting, the triac will fire at a frequency equal to twice the input AC voltage frequency). An electromagnetic interference (EMI) filter 14 filters high frequency signals and rf noise (e.g., harmonics) generated by the ballast, thereby preventing the conduction of such noise to the AC input source 10. An AC rectifier circuit 16 rectifies the input AC power to provide rectified DC power. The rectified DC power is coupled via a DC coupler 18 to a DC energy storage device 20. The DC energy storage device 20 maintains an DC voltage which is relatively higher than the peak of the rectified voltage output by the rectifier 16.
An inverter 22 converts the high voltage DC voltage stored in the DC storage device 20 to a high frequency voltage having a frequency which typically varies between about 20 and 75 Khz. The inverter 22 typically comprises two transistors forming a high frequency half-bridge inverter having an inverter output node. A DC blocking device 24 couples the high frequency output of the inverter 22 to a resonant circuit 26. As is understood by those skilled in the art, the resonant circuit 26, which typically includes at least one resonant inductor, resonant capacitor and feedback capacitor, is arranged to resonate at a frequency somewhat lower than the normal range of the high frequency voltage. The DC blocking device 24 is provided to prevent a DC bias component (e.g., the average of the high frequency square voltage waveform generated by inverter 22) from being applied to the resonant circuit 26 and saturating, e.g., the resonant choke. A load 28 (e.g. a fluorescent lamp) is magnetically coupled to the resonant circuit 26 via an output transformer (not shown). The DC blocking device 24 also prevents the DC component from being applied to the load 28 (which reduces the life of the lamps).
A feedback loop connects the resonant circuit 26 to a feedback node in the DC coupling device 18. During a portion of every high frequency cycle, current is drawn from the rectifier 16. In addition, during another portion of the high frequency cycle, charging current flows to the DC storage device 20. During the entire cycle of the input AC voltage, DC energy stored in DC storage device 20 is greater than the peak voltage of the rectified AC voltage from the rectifier 16.
Typically, at least one parameter (voltage or current) is sensed in the resonant circuit for providing suitable frequency modulation of the inverter 22 via a driver controller device 30. This sensed parameter may be used, for example, ensuring that the switches operate in zero voltage switching mode (inductive mode) so as to minimize switching losses.
As is understood by those skilled in the art, the inverter is typically implemented with a pair of Mosfets in a half-bridge inverter configuration. These Mosfet switches each have a parasitic capacitance c.sub.ds and parasitic resistance r.sub.ds between the drain and source. When a switch is turned off after having high voltage applied across its drain and source terminals, the parasitic capacitance is charged up to the voltage across the drain/source junction. When the switch is subsequently activated, the voltage across the parasitic capacitance c.sub.ds may be discharged through the parasitic resistance r.sub.ds, of the drain/source junction. When the inverter is operating at the steady state frequency of about 45-50 Khz, this current discharge may cause substantial losses unless the parasitic capacitor C.sub.ds is discharged through the body diode (from the substrate to the drain) prior to the switch turning on. Accordingly, prior to turning on the switch, current should be applied to the switch in the direction opposite the direction of the flow of current which occurs upon activating the switch. In the electronic ballasts shown in FIG. 1, when the triac fires, the resulting input ballast voltage and current causes an imbalance on the steady state voltages on the DC blocking device 24 (which is typically a capacitor). This voltage imbalance prevents the parasitic capacitances of the inverter switches from being discharged through the body diode prior to being activated, and results in the parasitic capacitances being discharged via the parasitic resistance r.sub.ds through the drain/source junction when the switch is activated.
This is illustrated with reference to FIG. 3. In FIG. 3, waveform A represents the input ballast current that is generated when the triac fires, and waveform B represents the drain current of a Mosfet switch which is generated for a plurality of cycles of the high frequency waveform. The negative current spikes of the B waveform indicate "zero voltage switching" in which the negative current flowing through the body diode of the switch results in a discharge of the parasitic capacitance. As shown in FIG. 3, one problem associated with the conventional triac dimmable ballast circuit discussed above is that, when the triac fires, the negative discharging of the switch drain current (waveform B) is lost for several cycles of the high frequency signal. Consequently, for these cycles, switching losses could be extremely dangerous to the switches at such high voltages since the charge of the parasitic capacitance is discharged through the drain/source junction via the parasitic resistance r.sub.ds, when the switch is activated.
Referring to FIG. 4a, a diagram illustrates a comparison between the inverter switching voltage and resonant inductor current waveforms in response to a step change of voltage provided by a triac in the conventional ballast circuit. As is shown, when a step change voltage is applied to the input, the resonant inductor current is shifted above the zero reference line, indicating a DC bias applied in the resonant current. This reason for this is related to the charging process of the DC blocking device 24 (capacitor). After the step up voltage is applied to the input (i.e, the triac fires), the DC blocking capacitor is charged by the input current through the feedback path, which creates a DC bias in the resonant inductor current, thereby causing the inverter to operate in a capacitive mode and preventing the discharge of the parasitic capacitance through the body diode of the switch.